A gap-repair insertion was used to duplicate exons 3 and 4. Analysis of the mutant cDNA showed a duplication of exons 3 and 4 as well as the excision of the neomycin selection gene and the plasmid sequence. Northern blot analysis demonstrated normal expression levels of mutant transcript. No protein was detected by immunoblots in isolates from liver mitochondria of homozygous mutant animals.